Test with live malaria parasites bolsters vaccine hopes

About malaria • Millions in the tropical and subtropical world are infected with malaria each year. • About a million die. • There is no vaccine yet, and preventive drugs are too expensive for most people.

In a daring experiment in Europe, scientists used mosquitoes as flying needles to deliver a "vaccine" of live malaria parasites through their bites.

The results were astounding: Everyone in the vaccine group acquired immunity to malaria; everyone in a nonvaccinated comparison group did not, and developed malaria when exposed to the parasites later.

The study was only a small proof-of-principle test, and its approach is not practical on a large scale. However, it shows that scientists may finally be on the right track to developing an effective vaccine against one of mankind's top killers. A vaccine that uses modified live parasites just entered human testing.

"Malaria vaccines are moving from the laboratory into the real world," Dr. Carlos Campbell wrote in an editorial accompanying the study in today's New England Journal of Medicine. He works for PATH, the Program for Appropriate Technology in Health, a Seattle-based global health foundation.

The new study "reminds us that the whole malaria parasite is the most potent immunizing" agent, even though it is harder to develop a vaccine this way and other leading candidates take a different approach, he wrote.

Malaria kills nearly a million people each year, mostly children under 5 and especially in Africa. Infected mosquitoes inject immature malaria parasites into the skin when they bite; these travel to the liver where they mature and multiply. Then they enter the bloodstream and attack red blood cells — the phase that makes people sick.

People can develop immunity to malaria if exposed to it many times. The drug chloroquine can kill parasites in the final bloodstream phase, when they are most dangerous.

Scientists tried to take advantage of these two factors, by using chloroquine to protect people while gradually exposing them to malaria parasites and letting immunity develop.

They assigned 10 volunteers to a "vaccine" group and five others to a comparison group. All were given chloroquine for three months, and exposed once a month to about a dozen mosquitoes — malaria-infected ones in the vaccine group and non-infected mosquitoes in the comparison group.

That was to allow the "vaccine" effect to develop. Next came a test to see if it was working.

All 15 stopped taking chloroquine. Two months later, all were bitten by malaria-infected mosquitoes. None of the 10 in the vaccine group developed parasites in their bloodstreams; all five in the comparison group did.

The study was done in a lab at Radboud University in Nijmegen, the Netherlands, and was funded by two foundations and a French government grant.

"This is not a vaccine" as in a commercial product, but a way to show how whole parasites can be used like a vaccine to protect against disease, said researcher Dr. Robert Sauerwein.

Two other reports in the New England Journal show that resistance is growing to artemisinin, the main drug used against malaria in the many areas where chloroquine is no longer effective, underscoring the urgent need to develop a vaccine.